Is it a sure thing that chemotherapy will work?

No. The chance of chemotherapy working depends on a number of factors including -- tumor type, stage, prognostic factors as well as host (patient) characteristics such as age, genetics, etc.
Not a sure thing. Our body is very complex and so is cancer cell. Respond to chemo not only will depend on complex physiology of our body but also on the complex biology of the cancer. Our absorption/ metabolism /clearance of the drug will affect both the respond and possible side efect. Things that are supposedly work in animals or test tube many times do not translate very well in human body.